The present invention relates to tunable dielectric thin films, and more particularly relates to strain-relieved and defect-reduced tunable dielectric thin films which significantly reduce dielectric loss at microwave frequencies.
Considerations in the development of tunable microwave devices based on ferroelectric materials are the dielectric constant, tunability and the dielectric quality factor (Q=1/tan xcex4) of the materials. The DC electric field dependent dielectric constant of ferroelectric thin films, such as Ba1-xSrXTiO3 (BST, 0xe2x89xa6xxe2x89xa61), is currently being used to develop low loss tunable microwave devices, such as voltage-controlled oscillators, tunable filters and phase shifters. This results from disadvantages associated with currently available tunable microwave devices based on PIN diodes and ferrites. Current semiconductor-based devices exhibit substantial losses at frequencies over 2 GHz, and high power is needed to operate current ferrite-based devices.
The provision of low loss tunable microwave devices based on ferroelectric thin films would reduce the size and operating power of devices while providing wide bandwidth and narrow beamwidth. One of the most critical properties that should be maximized in these applications is the dielectric quality factor of the ferroelectric thin films while maintaining a reasonable change in the dielectric constant with low DC electric fields at high frequencies (i.e., xe2x89xa72 GHz).
Although attempts have been made at developing tunable dielectric thin films having low dielectric losses, and improvements have been made at low frequencies (xe2x89xa61 MHz), prior art methods are not conducive to the use of these films in tunable applications at high frequencies (i.e., xe2x89xa72 GHz). Tunable dielectric materials having significantly increased Q values at low frequencies (xe2x89xa61 MHz) developed in the prior art are not commercially viable because currently available semiconductor materials have much better performance at those frequencies than the conventionally developed dielectric materials. A dielectric thin film ferroelectric device with optimal characteristics for tunable microwave applications has not yet been provided in the prior art.
The present invention provides dielectric thin films for applications such as electronically tunable devices having tuning specified for each device (i.e., voltage-controlled oscillators, tunable filters, phase shifters, etc.) and a high quality factor at high frequencies (xe2x89xa72 GHz). A strain-relieved tunable dielectric thin film is provided which minimizes a strain-enhanced inverse relationship between dielectric tuning and dielectric Q. The present invention provides strain-relieved and defect-reduced dielectric films that exhibit desirable dielectric properties at high frequencies (i.e., xe2x89xa72 GHz), which can be used in applications such as tunable microwave devices. The present invention also provides for annealing of the film material without thermally induced unit cell distortion caused by film strain due to thermal expansion mismatch between the film and substrate.
A process in accordance with an embodiment of the present invention includes the steps of: (i) forming a thin (e.g.,  less than 1,000 xc3x85) buffer layer such as BST (i.e., any porous phase between partially crystallized amorphous phase and fully crystallized randomly oriented phase) on a crystalline, low dielectric loss substrate by a low temperature deposition technique and a subsequent heat treatment; (ii) depositing a second layer (e.g., 5,000 xc3x85) of highly crystallized randomly oriented BST film on top of the BST buffer layer at a high temperature (e.g., 750xc2x0 C.); and (iii) annealing the film to reduce deposition-related crystalline defects (e.g., oxygen vacancies) and to grow crystalline grains.
In accordance with an embodiment of the present invention, the thin BST buffer layer relieves the film strain caused by film/substrate mismatches, i.e., lattice and thermal expansion mismatches between the film and substrate during the film deposition process and post-annealing process. The strain-relieved and defect-reduced tunable dielectric thin film according to the invention provides higher dielectric quality factors at high frequencies (xe2x89xa72 GHz) than currently existing prior art semiconductor materials as well as other prior art ferroelectric materials. Thus, the present invention provides a tunable dielectric thin film for tunable microwave applications, and a method of formation thereof, which relieves film strain having a significant effect on both microstructure and microwave dielectric properties of the film.
An aspect of the present invention is to provide a tunable dielectric thin film comprising a low dielectric loss substrate, a buffer layer on the low dielectric loss substrate, and a crystalline dielectric film on the buffer layer.
Another aspect of the present invention is to provide a method of making a thin film dielectric material. The method comprises the steps of depositing a thin dielectric buffer layer on a low dielectric loss substrate at a first temperature, and depositing a layer of dielectric thin film on the dielectric buffer layer at a second temperature. The deposited layers may be annealed to reduce crystalline defects and to grow crystalline grains.
These and other aspects of the present invention will be more apparent from the following description.